1. Field of the Invention
This invention relates to an optical scanning apparatus adapted to scan a surface to be scanned by means of a plurality of fluxes of light and also to an image forming apparatus such as a laser beam printer or a digital copying machine and an electrophotographic printer using such an optical scanning apparatus.
2. Related Background Art
In many conventional optical scanning apparatus that are used in image forming apparatus, the flux of light emitted from a light source is led to a deflection means by way of an optical means for incident light and the flux of light deflected by the deflection means is focussed on the surface to be scanned to form a spot of light there, which is then used to scan the surface by a scanning optical means.
In the course of technological development in recent years in the field of image forming apparatus, seeking for higher performances and higher functions, there has been and still is a strong demand for optical scanning apparatus that are compact and operate at high speed. An idea for meeting the demand for high speed operation is the use of a plurality of light sources. On the other hand, the demand for down-sizing may be met by converging the flux of light to be led to the deflection means along the principal scanning direction of the scanning optical means.
Japanese Patent Application Laid-Open No. 9-54263 discloses an optical scanning apparatus proposed to meet the demand for high speed operation. In the proposed optical scanning apparatus, a plurality of fluxes of light are emitted in a linear array from a single multi-beam laser chip. Japanese Patent Application Laid-Open No. 5-45580 discloses an optical scanning apparatus proposed to meet the demand for down-sizing. In the proposed optical scanning apparatus, the flux of light emitted from a light source is converged to a point of spontaneous convergence located behind the surface to be scanned by means of a focussing optical system.
However, optical scanning apparatus using a plurality of light sources are accompanied by the problem that the scanning start points of the fluxes of light from the light sources can be displaced from each other. While this problem may be dissolved by providing different time lags for the synchronizing signal to be applied to the light sources in order to cause the latter to start their respective scanning operations with the time lags. However, the time lags will have to be provided electrically or by means of software for each light source to make the overall configuration very complex and limit the scanning speed of the apparatus.
On the other hand, optical scanning apparatus designed to converge the fluxes of light led to the deflection means along the principal scanning direction utilize a rotary polyhedral mirror having its center of rotation located off the deflection plane as deflection means. If the fluxes of light from the optical means for incident light are convergent or divergent in such an arrangement, while the scanning start points can be aligned along the principal scanning direction, the deflection points may be shifted relative to the optical axis of the optical means for incident light to shift the points of spontaneous convergence as the rotary polyhedral mirror rotate. Thus, an image forming apparatus comprising such an optical scanning apparatus may have the spot images of the light sources displaced relative to the principal scanning direction as the rotary mirror rotates to make it impossible to form a high quality image.
In view of the technological problems of the prior art, it is therefore the object of the present invention to provide an optical scanning apparatus that can prevent any displacement of the scanning start points of the plurality of fluxes of light emitted from so many light sources from occurring by a simple means and also to provide an image forming apparatus and an electrophotographic printer comprising such an optical scanning apparatus.
In an aspect of the present invention, the above object is achieved by providing an optical scanning apparatus for scanning a surface to be scanned with a plurality of fluxes of light along a principal scanning direction, the apparatus comprising:
a plurality of light sources arranged at intervals along an auxiliary scanning direction perpendicularly to the principal scanning direction;
an optical deflector for deflecting the plurality of fluxes of light emitted from the plurality of light sources;
an optical system for incident light adapted to lead the plurality of fluxes of light emitted from the plurality of light sources to the optical deflector;
an imaging optical system for forming images of the plurality of fluxes of light deflected by the optical deflector on the surface to be scanned;
a photodetector for detecting part of the plurality of fluxes of light deflected by the optical deflector and outputting a synchronizing signal for controlling the timing of the scanning start of each of the fluxes of light; and
a slit arranged between the optical deflector and the photodetector;
the slit being inclined relative to the auxiliary scanning direction.
In another aspect of the invention, there is provided an image forming apparatus for forming an image by scanning the surface to be imaged with a plurality of fluxes of light along a principal scanning direction, the apparatus comprising:
a plurality of light sources arranged at intervals along an auxiliary scanning direction perpendicular to the principal scanning direction;
a central processing unit for driving the plurality of light sources independently relative to each other according to an image signal;
an optical deflector for deflecting the plurality of fluxes of light emitted from the plurality of light sources;
an optical system for incident light adapted to lead the plurality of fluxes of light emitted from the plurality of light sources to the optical deflector;
an imaging optical system for forming images of the plurality of fluxes of light deflected by the optical deflector on the surface to be imaged;
a photodetector for detecting part of the plurality of fluxes of light deflected by the optical deflector and outputting a synchronizing signal for controlling the timing of the scanning start of each of the fluxes of light; and
a slit arranged between the optical deflector and the photodetector;
the slit being inclined relative to the auxiliary scanning direction.
In still another aspect of the invention, there is provided an electrophotographic printer for forming an image by scanning the surface of a photosensitive body with a plurality of fluxes of light along a principal scanning direction, the apparatus comprising:
a plurality of light sources arranged at intervals along an auxiliary scanning direction perpendicular to the principal scanning direction;
a central processing unit for driving the plurality of light sources independently relative to each other according to an image signal;
an optical deflector for deflecting the plurality of fluxes of light emitted from the plurality of light sources;
an optical system for incident light adapted to lead the plurality of fluxes of light emitted from the plurality of light sources to the optical deflector;
a photodetector for detecting part of the plurality of fluxes of light deflected by the optical deflector and outputting a synchronizing signal for controlling the timing of the scanning start of each of the fluxes of light;
a slit arranged between the optical deflector and the photodetector, the slit being inclined relative to the auxiliary scanning direction;
a developing unit for developing an electrostatic latent image formed by scanning the surface of the photosensitive body with the plurality of fluxes of light;
an image transfer unit for transferring the developed toner image onto paper;
a fixing unit for fixing the toner image transferred on the paper.